The present invention relates to apparatus and methods for improving the heating efficiency of fireplaces, and more particularly, to a heat extractor insert for a decorative, non-wood burning fireplace whose flame is normally generated by combustible natural gas supplied thereto from utilities.
The heating function of fireplaces has been relatively unimportant in modern homes because of the widespread use of more effective alternate heating sources such as centrally located forced air furnace systems. Recently, excessive demand on sources of energy has resulted in continually escalating fuel costs. Increased attention has been focused upon improving the heating efficiency of fireplaces in terms of heat output versus fuel consumption.
Heat output from conventional fireplaces exists in three different forms: (1) conduction; (2) convection; and (3) radiation. In a typical openfront fireplace equipped with a grate substantially all of the heating effect results from radiant heat. Very little useful heat is gained by conduction or convection. In many cases, warm room air is actually drawn into the fireplace and exhausted up the chimney due to the rather substantial draft created by the fire. Since this creates a slight negative pressure within the room, cold air from outside the dwelling is drawn into the house through cracks and other inlets and this often results in a net loss of warm air from the room. While individuals sitting directly in front of the fireplace may be warmed by the radiant heat from a hot fire burning therein, such radiant heat does not warm the air as it travels therethrough. Thus, as soon as individuals remove themselves a sufficient distance from the fire, they sacrifice any warming benefits derived therefrom.
Another drawback of conventional fireplaces is that the wood is combusted too rapidly. Not only does this create a substantial draft up the chimney which draws warm air out of the room, but it also results in the expense of burning large quantities of wood with little or no effective heat output.
In an effort to reduce the loss of warm room air through open-front fireplaces, glass door closures have been used. Some of these have been equipped with adjustable dampers to permit the rate of combustion of the wood to be controlled.
In the interest of improving the heating efficiency of open-front fireplaces, tubular fireplace grates and other heat exchanger conduit structures for fireplaces have been developed. When the conduits are heated by the flames, cold air can be drawn into the tubes by convection and after being warmed, the air is expelled back into the room. Some of these heat exchangers have been equipped with blowers to increase their heat exchange efficiency. These heat exchangers can substantially increase the room air temperature.
Recently the desirable features of a glass door closure and a heat exchanger conduit structure have been combined into a single decorative fireplace insert. Products of this type reduce room air losses through the fireplace, while increasing convective heat output through the heat exchanger conduits and from the face of the glass door closure. They also permit wood to be combused at a slow controlled rate so that the maximum BTU value of the fuel can be extracted in an efficient manner. One such combination glass door closure and heat exchanger fireplace insert is disclosed in U.S. Pat. No. 4,129,113 of Bergstrom, and is sold under the trademark "THERMOGRATE". That insert includes a plurality of generally parallel C-shaped heat exchanger tubes having open lower and upper ends which draw and expel air, respectively, through a glass door fireplace closure. A blower forces air through a special manifold structure and through the tubes to maximize transfer of heat from burning logs surrounded by the tubes. In one commercial form of this insert, a sheet metal panel is mounted behind the tubes for redirecting heat normally lost to the rear of the fire chamber back onto the heat transfer surfaces of the heat exchanger tubes.
Heretofore, combination glass door closures and heat exchanger conduit inserts have generally been adapted for burning wood, although in some instances coal and other articulate fuels made of wood chips held together with a binder can be burned in such units. In some areas of the United States, particularly the Southwest, the high cost of fire wood prohibits the regular burning thereof in fireplaces. However, because of the aesthetic appeal of fireplaces, many homes in the Southwest are equipped with either masonry or sheet metal fireplaces equipped with natural gas outlets and asbestos logs. These gas nozzles are connected to gas supply lines in the home which are in turn connected to the gas supply lines of utility companys. The home owner can turn a key operated valve to cause natural gas, i.e., butane or propane, to be discharged from the nozzle within the fireplace and ignited. However, such gas fired fireplaces are not equipped with glass doors. Very litte effective heat is delivered to the room as a vast majority of the heat from the combusted gas flows up the chimney drawing warm room air out of the dwelling with it. Because of this, such gas fired fireplaces are ignited on an infrequent basis, for example during entertaining. The radiant heat of the fire produced in such a gas fired fireplace is typically not very great. It is not economic to burn combustible gas in such fireplaces at their high consumption rates without getting some reasonable return in the form of effective room heating. Therefore, it would be desirable to provide a gas fired heat extractor insert for the cavity of a gas fired fireplace which utilizes the advantages of a glass door closure and a heat exchanger conduit structure to provide effective room heat while still enabling the aesthetic benefits of the fire to be enjoyed. Such a combination insert would preferably have a high heat extraction efficiency, both to maximize room heating during colder climatic conditions, and to minimize fire hazard due to heating of rear and side dwelling walls.